1. Field of Invention
The invention relates to an electrophotographic image forming apparatus.
2. Description of Related Art
A known electrophotographic image forming apparatus employs a nonmagnetic single-component development system. In such an electrophotographic image forming apparatus as disclosed in, for example, U.S. Pat. No. 6,456,813, toner contained in a toner chamber is supplied by an agitator to a supply roller. The toner supplied to the supply roller is then supplied to a developing roller by the rotation of the supply roller. The toner is charged by friction between the supply roller and the developing roller. The toner supplied to a surface of the developing roller enters between the developing roller and a layer thickness regulating blade, as the developing roller rotates. The toner is sufficiently charged between the layer thickness regulating blade and the developing roller, and is formed into a uniform-thickness thin layer on the developing roller.
The toner carried as a thin layer on the surface of the developing roller is attached to an electrostatic latent image formed on a surface of a photosensitive drum, to form a visible toner image. The toner attached to the photosensitive drum is transferred onto a sheet by a transfer roller. Thus, an image is formed onto the sheet.
When the toner is rubbed between the layer thickness regulating blade and the developing roller, the toner is charged. At the same time, the toner is formed into the thin layer. As the toner is thus charged, the regulating blade accumulates thereon a charge with a polarity opposite to that of the toner. Such a charge opposite in polarity to the toner that has been charged with the proper polarity, is referred to as a countercharge. Due to the countercharge, the toner is attracted to the regulating blade. Accordingly, the properly charged toner cannot pass by the regulating blade.
The toner that happens to be charged with a reverse polarity, readily passes by the regulating blade due to the countercharge. This causes a deterioration in image quality. The countercharge accumulated on the regulating blade may be passed to the toner with poor charging performances, leading to unstable thin toner layer formation, as well as unstable toner charging.
The regulating blade may be firmly pressed against the developing roller, to increase frictional force and to frictionally charge the toner. However, this arrangement may cause the regulating blade to be worn quickly, leading to a deterioration in image quality.